High power LEDs are the conventional choice for general solid state lighting applications. Such high power white LEDs are extremely bright and can have luminous efficacies between 100 and 200 lumens/watt. The input power of a single high-power LED is typically greater than 0.5 watt and may be greater than 10 watts. Such LEDs generate considerable heat since they are only about 1 mm2 in area, so the required packaging is fairly complex and expensive. Although a bare high-power LED chip typically costs well under $1.00 (e.g., $0.10), the packaged LED typically costs around $1.50-$3.00. This makes a high output (e.g., 3000+ lumens) solid state luminaire relatively expensive and not a commercially feasible alternative for a standard 2×4 foot fluorescent light fixture, commonly used in offices. Further, the optics required to convert the high brightness point sources into a substantially homogeneous, broad angle emission for an office environment (where glare control is important) is extremely challenging.
To greatly reduce the cost of a large area, high lumen output light source, it is known to sandwich an array of bare LED dice between a bottom sheet having conductors and a top transparent sheet having conductors. The LEDs have top and bottom electrodes that contact a set of conductors. When the conductors are energized, the LEDs emit light. The light sheet may be flexible.
The Japanese published application S61-198690 by Hiroshi (filed in 1985 and published on 3 Sep. 1986) describes a light sheet using a plastic transparent front substrate having thin wires formed on it. A bottom substrate also has thin wires formed on it. An array of bare LED chips with top and bottom electrodes is arranged on the bottom substrate, and the front substrate is adhesively secured over the LED chips. LED chips at the intersections of energized perpendicular wires emit light.
The Japanese published application H08-18105 by Hirohisa (filed in 1994 and published on 19 Jan. 1996) describes a light sheet using a transparent front substrate having transparent electrodes (ITO) connected to metal strips. A backside substrate has metal conductors arranged in strips. Bottom electrodes of bare LED chips are bonded to the metal conductors on the backside substrate, such as using solder paste and reflow. A stamped “epoxy hotmelt adhesive” is provided on the backside substrate surrounding the LED chips. A liquid epoxy molding resin then fills in the inner area within the epoxy hotmelt adhesive. The hotmelt adhesive is then softened, and the front substrate is then affixed over the LED chips using the hotmelt adhesive and the cured molding resin. Applying current to the perpendicular strips of metal conductors on the opposing substrates energizes an LED chip at the intersection of two conductors. In one embodiment, the front and backside conductors/electrodes are formed over the entire surface, so all the LED chips will be energized simultaneously for use as an illuminator.
U.S. Pat. No. 6,087,680 to Gramann (priority filing date 31 Jan. 1997, issued 11 Jul. 2000) describes a light sheet using “elastic plastic” top and bottom substrates. Thin metal conductor strips and electrodes are sputtered onto the substrates or deposited in other conventional ways. Bare LED chips are provided with top and bottom electrodes. A conductive adhesive is used to adhere the bottom electrodes of the LED chips to the bottom substrate electrodes. A “coupling medium” fills in the spaces between the LED chips and is used for increasing light extraction. The coupling medium may be a liquid adhesive such as epoxy, resin, or silicone. The top substrate is then affixed over the LED chips, where the adhesive coupling medium affixes the substrates together and encapsulates the LED chips. Gramann describes the top and bottom substrates being “a structured conducting foil being formed essentially of plastic” that is capable of “plastic or elastic deformation,” so the light sheet is flexible.
Various patents to Daniels et al. have been issued relating to the earlier light sheets described above. These include U.S. Pat. Nos. 7,217,956; 7,052,924; 7,259,030; 7,427,782; and 7,476,557. Daniels' basic process for forming a flexible light sheet is as follows. Bare LED chips having top and bottom electrodes are provided. A bottom substrate sheet is provided with metal conductor strips and electrodes. A hotmelt adhesive sheet is formed separately, and the LED chips are embedded into the adhesive sheet. A transparent top substrate sheet is provided with metal conductor strips leading to transparent ITO electrodes. The adhesive sheet, containing the LEDs, is sandwiched between the top and bottom substrates, and the three layers are laminated together using heat and pressure so that there is electrical contact between the LED chips' electrodes and the opposing substrate electrodes. The process is performed as a continuous roll-to-roll process. The roll is later cut for a particular application. The LED chips may be arranged in a pattern to create a sign, or the LED chips may be arranged in an array to provide illumination.
In an alternative Daniels process, described in U.S. Pat. No. 7,259,030, a bottom substrate has an adhesive conductive sheet over it, on which is laminated a double sided adhesive sheet with holes. The LEDs are then placed in the holes, and another conductive sheet is laminated over the double sided adhesive sheet. The top transparent substrate is then laminated over the conductive sheet. The LEDs are electrically bonded to the two conductive layers by a high pressure roller at the end of the lamination process so the LEDs are connected in parallel.
Problems with the above-described prior art include: 1) little or no consideration for removing heat from the LEDs; 2) excessive downward pressure on the LEDs during lamination; 3) total internal reflections (TIR) caused by differences in indices of refraction; 4) difficulty in providing phosphor over/around the LEDs to create white light; 5) no consideration for enabling the light sheet to be optically functional and aesthetically pleasing if one or more LEDs fail (e.g., shorts out); 6) unattractive non-uniformity of light and color over the light sheet area; 7) difficulty of manufacture; 8) unreliability of LED electrode bonding; 9) excessively high lamination pressures needed to create wide light sheets; 10) inefficiency due to light absorption; 11) difficulty in creating series strings of LEDs; 12) impractical electrical drive requirements for the LEDs; and 13) inability of the light sheet to emit light in other than a Lambertian pattern. There are other drawbacks with the above-described light sheets.
What is needed is a cost-effective light sheet that can substitute for a standard fluorescent lamp fixture or that can be used for other lighting applications.